1. Field of the Invention
This invention relates to a water-containing adhesive based on glue and resin and to its use for bonding paper to plastic surfaces.
2. Discussion of Related Art
Glues are aqueous colloidal solutions of
a) animal polymers, for example casein and glutin, or polymers from skins, bones or fish, PA1 b) vegetable polymers, for example gum arabic, cellulose nitrate, carboxymethyl cellulose and cellulose ethers, dextrin and starch, PA1 c) synthetic polymers, such as polyvinyl pyrrolidone, polyacrylic acid derivatives and polyvinyl alcohol. PA1 A) a glue, PA1 B) a rosin, a synthetic resin and/or a derivative thereof and PA1 C) a monohydric or polyhydric alcohol.
Glues belong to the class of one-component, physically cold- or hot-setting adhesives in which the water escapes during the bonding process. Accordingly, they are mainly used for bonding absorbent materials, such as paper, cardboard or wood, to one another. However, glues are not suitable for permanently bonding paper, particularly paper labels, to plastic surfaces (see K. Dullinger "Etikettiertechnik von A bis Z" in Zeitschrift fur Lebensmitteltechnik (ZFL) 1987, pages 503 to 507).
It is also known that synthetic resin dispersions can be added to glues, for example starch and dextrin glues, to improve their resistance to water. Synthetic resin dispersions, for example polyvinyl esters and polyacrylates and also copolymers thereof, are also used either on their own or together with polyvinyl alcohol for bonding paper to plastic. The object of adding polyvinyl alcohol is to increase the wet adhesive strength of the dispersion. Fully synthetic systems such as these are eminently suitable for bonding paper to plastic surfaces (see Dullinger, loc. cit.). However, they are attended by the disadvantage that high wet adhesive strength can only be achieved by adding relatively large quantities of PVAC which, at the same time, distinctly reduces adhesion to plastic surfaces.
DE 40 00 241 A1 describes an adhesive of animal glue, glue hydrolyzate or acid casein and at least one substance from the group consisting of urea, thiourea, N-alkyl urea and guanidine nitrate and optionally other additives typically used in glue production. Gum rosin, sugar or dextrin may be added to improve the non-curl properties. Glycerol or polyglycol, for example, may be added as a hygroscopic substance. The adhesive may be used in the form of an aqueous solution or solid. The document in question claims the use of the adhesive as a heat-sealing material or hot adhesive, more particularly for surface coating.
Accordingly, the problem addressed by the present invention was to provide an adhesive which, similarly to glues, would be largely based on natural raw materials, but which would develop similar strengths to synthetic resin dispersions in the bonding of paper to plastic surfaces and would have a high wet adhesive strength.